LPL 4500 dying power supply?

Exposures on my LPL 4500 Dichroic have suddenly gone from the range of 10 seconds to 50+ even with no filtration in place. I've swapped bulbs, gone through the entire light path and I can't find anything wrong mechanically.
The light is visibly and substantially dimmer than normal (duh!).
I figure the power supply must have died, or is in the process of doing so.
I haven't worked out how to check the voltage with a load, with no load it's producing 120 V at the lamp.

Has anyone encountered this before?

Thanks
Barry

bdial said:

Has anyone encountered this before?

Click to expand...

Not so far, but I often wonder what I'll do to replace mine if it does die. Saunders is now Satter/Omega I think. I'll have to check their website to see if they sell replacements, or do repairs.

bdial said:

.....
I haven't worked out how to check the voltage with a load, with no load it's producing 120 V at the lamp......

Thanks
Barry

Click to expand...

Do you know how to use a voltmeter?

If you do and have one - try this.

Get a screw-in socket (i.e. it is a regular two or three-prong light receptacle that has a light bulb like screw thread so you can screw it into a light bulb socket).

Make sure you set your voltmeter to AC voltage measurement and within a range that covers 120v.

Plug a probe into each "slot" of the socket and see if you read 120v.

You don't give the rated lamp voltage but as it's a Dichroic head I assume it should be 24volt and the lamp wattage is either 200 or 250 watt. It could simply be that the lamp holder has burned contacts and needs replacing, ideally these should be replaced each time the lamp is changed, but this rarely happens, so overtime the contacts char. However the fact that you are getting an open circuit voltage of 120v rather than the expected 30 volt suggests a transformer problem. As to replacement, if that proves to be the need, then any 24 volt output transformer rated at 300 watts or greater should do. Obviously it also needs to have an input voltage that matches your supply voltage. If in doubt consult an electrician, and get the unit tested. I doubt that repair of the transformer will be economic.

I should have added to the above that these days it is sometimes cheaper to buy another secondhand enlarger than to pay for repairs to the original. Sad but true.

Barry, should it come to that, Omega/Satter carries a replacement unit for $575.00. YIKES! Still, that's a lot less expensive than a new enlarger (and LPL 4550 and 4500 models don't come up very often used.). If you check their site, and click on "online dealers" there's a long list, including Badger Graphics. Here's a link:

http://www.omegasatter.com/Product_Info.php?pid=3537&cid=73

John,
Thanks, yikes indeed, though I had already gone through that sticker shock with a visit to Omega Satter's site. B&H has one in stock for slightly less. It's way more than I paid for the entire enlarger, though I'm well aware that I got a really good deal. In purely objective terms, I could buy the new power supply and still be ahead over what a new or even a typical used 4500 runs. But my wallet isn't that objective, at least not yet.

Dave, the lamp is 200 watts and operates at 82 V it's the 82 v part that's really inconvenient. A 24 V supply would probably be fairly easy to cobble up, and (for us) 120 V would be a piece of cake, since I could just power it from the mains. I've anticipated this eventuality, but I was hoping it would be farther in the future.

With the emerging limitations on tungsten lights, it might make more sense to see what can be done to convert to LED, conveniently, the lamp is an MR16 base, so except for light output, and perhaps color temp, there are a lot of possibilities for creativity. For B&W, I suppose the color temp isn't too much of an issue. It's a dichro. head, so I probably have enough filtration to compensate for VC papers. I don't currently do any color, but I would prefer to not lose that ability.

I'm open to ideas.

Thanks
Barry

bdial said:

John,
the lamp is 200 watts and operates at 82 V

Click to expand...

That seems to be pretty standard on Dichro heads.

82V RMS is 1/2 wave rectified 117V. In cheaper, unregulated, heads there is just a diode to drop the voltage by cutting out the negative (or positive) AC cycles. When used in regulated or closed-loop heads the power to the lamp is controlled by a triac that fires at a nominal 90 degrees. At 90 degrees on the AC waveform controlling the average voltage does a reasonable job of controlling the RMS voltage. The voltage is actually 82.7V - but the diode (or triac) takes away 0.7 volt, leaving 82V to go to the lamp.

Measuring the voltage at the lamp holder with no lamp can result in any voltage between 58V and 117V showing up on the voltmeter. If the voltmeter draws only a microamp or so then the diode/triac will have no effect as it 'leaks' more current than this and the voltmeter will show full line voltage. If the voltmeter draws enough current that the diode is effective then it may read 58V if it displays the average voltage instead of the RMS voltage. A voltmeter that reads RMS will usually display this prominently on the front panel.

For reference: 117VAC RMS house voltage has a peak voltage of +/- 165V and a peak-to-peak voltage of 330V. RMS stands for 'Root Mean Square' and can be thought of as the heating power - 117V DC will light a lamp as brightly as 330V pk-pk AC, and so the 330 V pk-pk sine wave AC voltage at the electrical outlet is specified to be 117V, and the 'RMS' is left out/understood.

You would think that removing half the AC cycle would result in half the RMS voltage, but it doesn't work out that way - it is instead reduced by the square root of one half. The reason for this is that the heating power of the voltage is proportional to the square of the voltage: increasing the voltage by 10% puts 21% more power in to the lamp. This is the reason enlargers are so sensitive to power fluctuations - a 10% power dip is a 1/3 stop drop in light output (actually a bit worse because the lamp spectrum shifts to the red).

Dim lamp output is usually the result of a dirty/corroded lamp holder. Replace the holder and inspect/tighten all the wire connections and everything should be restored.

If you have a regulating power supply then it is possible the power supply is defective. $500 is a pretty creative price for what is essentially a modified lamp dimmer. If you don't mind not having regulation then a 10A 700V diode can stand in for the power supply.